Pretty Park begins on the first day of Spring as Miss Piggy decides to take Kermit on a picnic in the park near her house. She enlists the help of Rowlf, Fozzie and Gonzo to help her pick out a spot, but upon their arrival, they find that the park is in terrible shape. Everything was covered in litter, the benches and picnic tables had been vandalized and the swing sets were broken.

Disgusted and frustrated, Piggy complains to the park ranger who admits that people make a mess of things quicker than she can clean it all up. Piggy and her friends decide to help in the ranger's efforts, and organize Operation Pretty Park. The next morning, they all show up with tools to repair, trash bags and flowering plants. After some effort, the team of friends have bundled up all the garbage, placed trash and recycle bins in the park and brought color back in the form of flourishing flower beds.

The next weekend, Piggy is finally able to take Kermit on a picnic for two. The park is filled with people enjoying the clean paths, using the swings and relaxing on benches. They all notice how pretty the park is and figure that they ought to try and keep it that way.

The book ends with a message from Kermit about what can be done to keep neighborhood parks clean. Suggestions include placing litter in trash cans, refraining from picking flowers and getting involved in local environmental groups or becoming an honorary park ranger.